Modelling regional sea-level changes in recent past and future

Abstract

Sea-level change is one of the most important consequences of a warming climate, affecting many densely populated coastal communities. To improve coastal management and the planning of flood defences, information on the future development of sea-level rise is needed. However, sea-level rise is not uniform around the world. It is therefore not sufficient to know how much the global mean sea level will rise in the future. Instead, there is a pressing need for information on a regional scale. Making sea-level projections, both globally and locally, requires understanding of the processes that contribute to sea-level change. The research in this thesis focuses on modelling these processes, and in particular on their regional patterns in sea-level change. Firstly, sea level can rise or fall due to the addition or removal of water. Water may be added when glaciers or ice sheets shrink or due to groundwater pumping, which may be partly compensated by storage of water in newly constructed dams in rivers. Apart from the direct effect of addition or removal of water, a gravitational effect needs to be considered when large masses of water are displaced. This gravitational effect causes a very distinctive pattern in sea-level change, with a sea-level fall close to the water input source, and sea-level rise further away from the source. Apart from the gravitational effect, there is a reaction of the solid earth to changes in the redistribution of mass on the Earth’s surface. This is an effect that occurs immediately, the ‘elastic’ response, as well as on time scales of thousands of years, the ‘viscous’ response. The latter process, termed ‘Glacial Isostatic Adjustment’, results for instance in an uplift of up to a centimetre per year in parts of Scandinavia, as a result of ice melt after the Last Ice Age. Another cause of sea-level change are density variations, which occur when temperature or salinity changes. An increasing temperature causes expansion and thus sea-level rise, while increasing salinity causes densification and thus sea-level fall. Since these changes are not the same everywhere on earth, this causes a very irregular pattern of sea-level change. The research described in this thesis combines the knowledge on these processes to produce regional patterns of sea-level change. These patterns have been computed for the period 1961-2003, and compared to sea-level measurements over the same period in order to see whether we understand the regional patterns in sea-level change. The same approach is also used for future projections. These projections clearly show that regional variations are expected to be substantial. We find that 10% of the ocean surface will experience a change that differs more than 25% from the global mean. Our research also shows that each process may locally dominate sea-level change, and hence it is very important to include all these processes when considering regional variations.